A recently developed technique of tracking motion and accumulating position by correlating shifted instances of a previous image with new images of surface micro-texture, such as the fibers in a sheet of paper, offers ease of use and outstanding accuracy. A generic term for such a technique is "optical navigation." A hand held scanner and a mouse for a computer are among the devices that can use optical navigation, and examples are described in the incorporated patent documents.
Optical navigation techniques require that an illumination source (e.g. an Infra Red Light Emitting Diode, or IR LED) be used to create contrast in an image of the surface. To assist in the creation of shadows that reveal texture, the surface is generally illuminated from the side, and the result is often termed "grazing" illumination. The amount of reflected illumination reaching the photo sensitive detectors is termed "exposure." Exposure can be controlled in different ways. Among these are: controlling the open time of an electronic shutter while maintaining a constant level of incident illumination; controlling the intensity of a pulse of light issued while the electronic shutter is open (assuming there is one); and controlling the duration of a pulse of light occurring within the open time of an electronic shutter (again, assuming there is one).
Exposure varies considerably among different surfaces. Consider paper, which is perhaps the most frequently used surface. The type of fiber in a paper and the roughness and color of the paper, as well as information content related conditions like the presence of ink or fused toner, are all items that affect the amount of light incident upon the surface needed to create a certain level of exposure. Insufficient exposure fails to reveal contrast in the micro-texture ( the image sensor has a certain sensitivity that must be met), while over exposure "washes out" the image (and is either a saturation issue or a dynamic range limitation: the difference between bright and too bright). As an example of this phenomenon, consider the following table of different optimum exposure times at a constant level of incident illumination for different papers also having different image properties:
EXPOSURE TIMES (uSec) PAPER TYPE A 10 2 PAPER TYPE B 20 4 PAPER TYPE C 30 8 PAPER TYPE D 40 10 LOW VARIANCE HIGH VARIANCE IMAGE IMAGE
It has been found that successful optical navigation over surfaces having differing properties requires that the illumination exposure be varied according to the conditions encountered. However, the task of adjusting the exposure during navigation as conditions vary is further complicated by the nature of the navigation process. It compares a reference image with successive images of micro-texture through a correlation process to determine the direction and amount of movement. After sufficient movement a new reference image is acquired. There is a high risk of compromising the comparisons if the level of incident illumination is allowed to vary at times other than when a reference image is being acquired.
It has also been found that simply adjusting the level of incident light to obtain some preselected median level of reflected light may not provide sufficient contrast for accurate navigation. A more sophisticated approach is required if optical navigation techniques are to more reliably operate over surfaces exhibiting widely varying surface conditions.